The present invention relates to electronic test and measurement systems in general, and in particular to modular electronic instrument systems having automated calibration capability.
Calibration of electronic test and measurement instruments in regular intervals is essential for maintaining proper operation and accuracy. Because values of internal components of such instruments change with age, temperature, and other effects, including catastrophic failure and replacement, recalibration of instruments is required from time to time. The periods of time between recalibration may vary, depending on instrument complexity, use, and other factors.
Recalibration of instruments has traditionally been a time consuming and somewhat tedious task, requiring that a calibration technician assemble proper calibration equipment and then follow a procedure in a manual for the particular instrument to be calibrated. He also has to keep detailed records of equipment model numbers and serial numbers, and the actions he performed. In large calibration depots or laboratories, technicians have often prepared their own calibration procedures, either to match equipment on hand to do the calibration, or to streamline the manufacturer's calibration procedure where possible.
Automated calibration systems, and, in some instances, self-calibrating microprocessor-based instruments, have become commercially available from a number of suppliers. Such automated calibration systems typically include a number of pieces of calibration equipment in a rack along with an instrument controller/computer interconnected by a common interface such as a standard IEEE-488 bus or the like. The calibration equipment comprises precise signal sources and sensors, e.g., signal generators and highly accurate digital multimeters, respectively, used as references for other instruments to be calibrated. The references themselves may be calibrated at predetermined intervals against even more accurate standards such as those which typically reside in a so called "primary standards laboratory" or are present at the National Bureau of Standards.
In prior art automated calibration systems, software controls the display of messages, and where applicable, provides the control of instruments. The instrument controller executes programs specifically written for the instrument to be calibrated, and generally matches the steps described in the manual for that target instrument. Thus the programs simply display messages to instruct a calibration technician to set front-panel controls on the instrument to be calibrated, hook up cables and test leads, control calibration instruments to provide stimulus or a measurement, and to verify operation or make circuit adjustments as required for calibrating a target instrument. After each step is completed, the technician presses a button to allow the calibration system to proceed to the next step. If the target instrument is remote controllable, the automated calibration system may supply an appropriate signal or voltage if instructed to do so at any given step, and to inform the technician whether the specified tolerance is met. Automated calibration systems also automatically keep records of calibration so that, if necessary, detailed information regarding the accuracy of that instrument is available. This is a requirement, for example, in the nuclear power industry. Additionally, some modern microprocessor-controlled target instruments may store correction constants in their memories instead of using variable resistors and capacitors to make necessary adjustments. The correction constants, which may be stored in the target instruments under remote control by the calibration system, may be used as multipliers for each of a number of ranges so that displayed readings are correct for each appropriate range. Examples of situations where correction constants may be appropriate include the voltage ranges of digital voltmeters and the vertical and horizontal deflection factors of oscilloscopes.
While the prior art automated calibration systems have greatly reduced the time required to calibrate instruments, and in many cases have allowed calibration to be carried out by lower skilled personnel, some major drawbacks yet exist. It is necessary that for every target instrument to be calibrated, a specific calibration procedure must be written which matches the requirements for calibration by the maintenance manual applied by the manufacturer. Some of the instructions to the operator can be standardized, such as function, range, and connection messages; however, instructions as to which variable resistor to adjust and within what tolerance must be explicitly created. Moreover, such instructions must be created in the program language for the system controller. Also, such instructions are rather specific as to which calibration instruments, by model number, are used in the system since they must be explicitly programmed to provide the appropriate stimulus or quantity to be measured.
As a rule, calibration instruments are not readily interchangeable among different models or among different manufacturers, and care must be taken to ensure that replacement equipment has the necessary functions and accuracy to perform the calibration and to operate properly in the calibration procedure. It is recognized that, as a rule, calibration equipment must be at least four times more accurate than the instrument to be calibrated, and it is typically very difficult to locate a single replacement for a calibration instrument which adequately provides the required accuracy, particularly if the calibration instrument is to provide several functions, each with different ranges and accuracies.